1. Field of the Invention
The present invention relates to an image pickup device, and more particularly to an image pickup device with a function to control the white balance of the video signal obtained from the output of image pickup means.
2. Related Background Art
In recent image pickup devices such as of video movie cameras and electronic still cameras, the white balance is being controlled by a method of automatically maintaining satisfactory white balance in the obtained video signal utilizing the output of the image pickup element, namely the TTL auto-tracking white balance control.
An example of such white balance control method will be briefly explained in the following. In such image pickup device as mentioned above, the signal obtained from the image pickup element is generally converted, through a color separation circuit and a process circuit already known in the art, into a Y (luminance) signal, a R (red) - Y signal and a B (blue) - Y signal. In this process, variable-gain amplifiers are generally provided for the R and B signals or for the (R-Y) and (B-Y) signals.
The Y, (R-Y) and (B-Y) signals are respectively averaged, for example by integration, over a period of a field or longer, and these averaged signals are used for deriving R, G and B signals. These three primary color signals are used for deriving control signals for controlling said variable-gain amplifiers in such a manner that the ratio of said three primary color signals becomes equal to 1:1:1.
This method enables automatic white balance control, without relying on particular color measuring sensors.
On the other hand, the above-explained method, utilizing the output of the image pickup element for the white balance control, is associated with a drawback of being incapable of satisfactory white balance control by the influence of object, in case an object of high saturation occupies a large area in the image field.
It is therefore already known to provide a clipping circuit in front of the above-mentioned averaging step, thereby alleviating the influence of the object of high saturation.
Such clipping circuit is anticipated to provide the expected effect in case, as shown in FIG. 1A, the high level side (100) and the low level side (200) are evenly clipped, but, if the high level side (100) alone is clipped while the low level side (200) is not clipped as shown in FIG. 1B, because of the color of light source or of a variation in color temperature, the white balance control is affected by the signal of said low level side. Naturally a similar result occurs when the low level side (200) alone is clipped.
More specifically, in movie cameras in which the time constant of the above-mentioned averaging circuits is selected relatively long in order to gradually reach the appropriate white balance state, there will be required a long time before reaching the appropriate white balance state. On the other hand, in case the appropriate white balance has to be reached within a short time as in the electronic still cameras, there is encountered a drawback that the phototaking operation is executed in a situation in which the white balance is not yet appropriate.
Also the aberration in white balance, resulting from the above-mentioned influence of object of high saturation, is not too conspicuous in case the color shift takes place corresponding to the variation in color temperature of black body radiation, namely in case of color shift toward red or blue, but becomes visually very unpleasant in case a color shift toward green or purple is involved.
Such visually unpleasant aberration of the white balance can be avoided by simply limiting the direction of color correction in the white balance control approximately to a direction corresponding to the variation of color temperature, but, in such case, the white balance control cannot be achieved under a light source which is greenish in comparison with the black body radiation, such as the fluorescent lamp.